Polystyrene foam is most commonly made by combining a physical blowing agent with molten polystyrene resin under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere. From about the 1950's to the present, the physical blowing agents of choice have been the halocarbons, hydrocarbons or mixtures of the same. Examples of these include commercially available Freon compositions, such as dichlorodifluoromethane, trichlorofluoromethane and the C.sub.4 -C.sub.6 hydrocarbons, with isopentane being preferred. These blowing agents, while being effective in producing the proper foam structure and excellent surface characteristics, do have their own drawbacks and disadvantages. These blowing agents are released to the atmosphere during and after foam production and are a cause of pollution. The hydrocarbon blowing agents in addition constitute a fire hazard.
Through the years, carbon dioxide has been experimented with as a blowing agent for polystyrene resin. It is listed as a blowing agent in Gilbert et al, U.S. Pat. No. 3,431,164, Noel, U.S. Pat. No. 4,198,363, Johnson, U.S. Pat. No. 4,470,938, and others. No prior investigator has actually demonstrated that 100% CO.sub.2, can produce a commercially acceptable foamed polystyrene product. To date, the best utility for carbon dioxide in foamed polystyrene is as a co-blowing agent or a diluent for hydrocarbon or halocarbon blowing agents, see, for example, Johnson et al U.S. Pat. No. 4,344,710 and the article by L. M. Zwolinski and F. J. Dwyer, Extruded Polystyrene Foam with CFC/Carbon Dioxide Blowing Agents, ANTEC'86 pages 30-33, Conference Proceedings, Society of Plastic Engineers 44th Annual Technical Conference and Exhibit.
It is an object of the present invention to provide a polystyrene resin in foam form employing as a precursor resin a polystyrene not heretofore successfully converted to an excellent foam structure.
It is another object of the present invention to present a process for foaming a particular class of polystyrene resins employing 100% carbon dioxide as the blowing agent.